Casings for gas or steam turbines are big and heavy, and must be attached gas-tight against each other. Conventionally, this is done by utilizing a number of set bolts arranged at a distance from each other in a circle or another geometric form adapted to the peripheral area of the casing, and which are prestressed by application of substantial force. In turbines, increasing steam or gas pressure results in improved efficiency and because of this, over the years increasing prestress has been applied.
The method most widely used today for prestressing bolts is working with so-called heating bolts, that is, bolts which are heated and mechanically stressed when hot. The time needed for heating and cooling of the bolts is substantial so that even under good circumstances as many as several days are needed for mounting a casing of a large gas or steam turbine.
It is necessary to wait until the bolts have cooled down before checking whether the correct prestress has been achieved. If correct prestress has not been achieved, the heating and cooling process must be repeated, something that will further increase the time spent.
During prestressing and securing of bolts in turbine casings, similar stresses arise within the casings themselves and will remain even after all bolts have been mounted. If the bolts are mechanically prestressed, such stress in a turbine casing may be reduced by simultaneously stressing bolts symmetrically arranged in respect of the center of the casing, or by stressing in small steps, whereby the prestress for each bolt will be increased essentially to the same extent and simultaneously. Such a procedure is impossible when using the hot prestressing method which, by necessity, results in inferior prestress.
Repair and maintenance work on large turbines requires them to be taken out of service which often is very expensive. Thus, a reducing of down time required for repair or maintenance is desirable, and may be an economic necessity.
For a long time hydraulic pretensioning of bolts has been known. The hydraulic pretensioning method is substantially faster and easier to reproduce than prestress obtained by heating. However, it has not been possible to use that method for bolts in turbine casings or similar equipment where space for utilizing necessary prestressing equipment in the traditional way is limited.